Electrostatic charging and image formation



Dec. 20, 1960 R. w. GUNDLACH ELECTROSTATIC CHARGING AND IMAGE FORMATION Filed Aug. 1, 1955 2 H 1 n L ME H R mmw T W H w (I Z M 2 v J "Y 1. Q L A m I T ME 1 E TEC IIEC OU O 6 SP EPO 0H0 Nm P H H Fig.1

H 16H VOLTAGE- s ouRclz INVENTOR. ROBERT W. GUNDLACH A TTORNE Y United States Patent 'Ofiice 2,965,481 Patented Dec. 20, 1960 ELECTROSTATIC CHARGING AND IMAGE FORMATION V Filed Aug. 1, 1955, Ser. No. 525,497 3 Claims. (Cl. 96-1) This invention relates in general to xerography and, more particularly, to the formation of electrostatic charge patterns on the surface of xerographic plates.

In xerography it is usual to form an electrostatic charge pattern on the surface of a xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member. The pattern is formed by first sensitizing the plate by placing an electrostatic charge on the surface of the photoconductive insulating layer. The charge is generally placed on the surface while the conductive backing support for the photoconductive insulating layer is maintained at ground potential, and the grounded backing member causes deposition on the surface of the photoconductive insulating layer of ions because of fields of force existing through the surface being charged. Following sensitization, the plate is generally exposed to an activating radiation pattern such as a light image or the like. The activating energy, such as the light energy, causes charge dissipation whereas areas not struck by the activating energy during exposure remain in their charged condition. Thus, after exposure an electrostatic image of electrostatic charges exists on the surface of the xerographic plate conforming to the pattern of radiation to which the sensitive plate was exposed. This pattern may be developed or otherwise utilized. When developed known techniques of development such as cascade, powder cloud, magnetic development, or the like, may be used or the charge pattern may be otherwise utilized such as transferred, scanned, or the like. A developed image on the plate surface may be fused, photographed, or the like, or it may be transferred to a transfer base on which it may be permanently aflixed.

This invention relates to image formation in xerography on plates comprising a self-supporting film of photoconductive insulating material. The plate used in this invention is characterized by its ability to be formed into a self-supporting fihn of material and by the normal xerographic requirements of rapid charge dissipation on exposure and ability to retain charge for a sufficiently long period to permit exposure and development.

It is an object of this invention to devise novel methods to improve the art of xerography.

It is a further object of this invention to devise a novel plate to be used in the art of xerography.

1 It is still another object of this invention to devise novel methods of charging of self-supporting films of photoconductive insulating layers. It is a still further object of this invention to improve upon methods for applying uniform electrostatic fields through a photoconductive insulating layer.

, It is a still further object of-this invention to devise novel methods of image formation on xerographic plates comprising self-supporting films of photoconductive insulating material.

Other objects and advantages of this invention will be apparent in view of the following description taken in the light of the accompanying drawings, wherein:

Fig. l is a diagrammatic view of one embodiment of charging according to this invention;

Fig. 2 is a diagrammatic view of another embodiment of charging according to this invention;

Fig. 3 is an embodiment of a charging device which may be used in the charging technique illustrated in Fig.1;

Fig. 4 is an embodiment of exposure according to this invention; and a Fig. 5 illustrates development of a charge pattern.

For a better understanding of this invention, reference is now had to Fig. 1 wherein is shown one technique of charging according to this invention. The plate desig nated 10 comprises a self-supporting film of a photoconductive insulating material and is, as is illustrated, being moved from right to left. Positioned above plate 10 is a corona discharge electrode generally designated 13 comprising a corona discharge wire 15 and a shield 16. Corona discharge wire 15 is connected to a negative polarity high potential source 17 and shield 16 is held at ground potential. Positioned on the rear side of plate 10 is a corona discharge electrode generally designated 18 comprising a corona discharge wire 20 and shield 21. Corona discharge wire 20 is supplied with positive polarity high potential from positive high potential source 22 and shield 21 is maintained at ground potential. As illustrated in this figure, charges of opposite sign are deposited on the opposite surfaces of plate 10 as the plate moves between corona discharge electrodes 13 and 18.

Reference is now had to Fig. 2 wherein another embodiment of charging according to this invention is illustrated. Plate 10 is in this embodiment positioned on a support table 11 which is a conductive material optionally maintained at ground potential. Positioned above plate 10 is a corona discharge electrode generally designated 23 which comprises a shield 25 desirably grounded and corona discharge wire 26. A high potential from high potential source 27 is supplied to corona discharge wire 26 of corona discharge electrode 23, and ions are supplied from corona discharge electrode 23 to the upper surface of plate 10. In this embodiment there is illustrated deposition of positive charge on the surface of plate 10 and, thus, high potential source 27 is acting as a supplier of positive polarity high potential. The corona discharge electrode 23 in this embodiment is moved at a uniform rate from left to right by motor 24 driving screw 29 connected to electrode 23 across the surface of the photoconductive insulating layer '10 to cause a uniform amount of charge deposition on the surface of this layer. Other drive means generally known to those skilled in the art may be used and are intended to be encompassed by this invention.

Reference is now had to Fig. 3 wherein an embodiment of a charging device is illustrated which may be used in the technique of charging described in connection with .Fig. 1. As in Fig. l, 15 represents the corona discharge wires of the upper corona discharge electrode and 20 rep resents the corona discharge wires of the lower discharge electrode. In this embodiment, the upper and lower corona discharge electrodes are connected together by end walls 46 and thus comprise a single unit. End walls 46 are sheets of insulating material such as Lucite, other insulating matic, or the like. It is to be realized of course thatthe end walls may com-prise other materials and may be constructed in various ways as will readily occur to those skilled in the art. Corona discharge wires 15 are connected together by connecting links 47, and corona discharge wires 20 are connected together by connecting links 48. Wires 15 are connected to the positive end of high voltage source50, and wires 20 are connected to the negative end of high voltage source 50. High voltage source.50 is'for the purpose of supplying the desired po- 3 tential for corona discharge to wires 15 and 20. It is to be realized, of course, that other known techniques of supplying a high potential to corona discharge wires 15 and 20 are intended to be encompassed by the scope of this invention.

At the upper end of the simultaneous charging corona discharge electrode of this figure is plate 52. A similar plate 53 is at the base of the device. Plates 52 and 53 are layers of conductive material and are grounded as illustrated. Extending downward from plate 52 are panels 55 and 56 of conductive material in electrical contact with plate 52. The combination of panels 55 and 56 and plate 52 partially surrounding corona discharge wires 15 may create What was discussed as the upper corona discharge electrode 13 of Fig. 11. The lower discharge electrode has extending upward from plate 53 panels 57 and 58 which are conductive materials electrically connected to plate 53. The two forward panels 56 and 58 are shaped at their ends as illustrated to provide guide means for a plate 10 to move therebetween. Panels 57 and 55 are also shaped at their ends to guide and position the plate 10 as it moves therebetween through the corona discharge electrode. Although a particular deviceis illustrated in this figure, it is to be realized that modifications may be made which will occur to those skilled in the art and such modifications are intended to be encompassed in this invention.

In Fig. 4 there is illustrated one technique of exposure which may be used with this invention. The plate 10 illustrated in this embodiment has been charged according to the method of charging illustrated in Fig. 1 and, thus, when placed in position for exposure carries a layer of positive charge on the lower surface surface and a layer of negative charge on the top surface. Positioned above the plate is copy 31 to be reproduced and lens 32 through which the copy is projected to the surface of the self supporting film of photoconductive insulating material 10 There is illustrated in this figure projection exposure. However, it is to be realized that other techniques of exposure generally known to the art such as contact, reflex, and the like are intended to be included herein. In this figure, beams 33 of light energy are shown projecting to the plate surface. Beams 33, on striking plate 10, cause charge dissipation in areas of the plate as illustrated; whereas those areas not struck by beams 33* continue to hold in position the original charges placed on the plate during the charging step.

Reference is now had to Fig. 5 wherein one technique of development is illustrated. Development as is illus trated in this figure is generally known to the art as magnetic development. Plate carrying a charge pattern formed during exposure of a sensitized plate is being developed through the use of a magnet 35 carrying electrostatically charged magnetic particles 36. The particles adhere to the magnet due to magnetic attraction. 'When passed across and in contact with the surface of the plate carrying an electrostatic charge pattern, the electrostatic attraction of the pattern overcomes the magnetic attraction of the magnet, and the particles deposit to develop the charge pattern. Other techniques of particle deposition generally known to the art as, for example, cascade development described in Walkup and Wise US. Patent 2,638,416, powder cloud development, and the like may be used and are intended to be encompassed by this invention. 7

Although a particular corona discharge electrode is illustrated, other corona discharge electrodes and other means of applying electrostatic charge are intended to be included herein. For example, in co-pending application Serial No. 154,295, a corona discharge electrode is described wherein a screen of wires is interposed between the surface to be charged and the corona discharge wire or wires. The screen in that application acts to regulate the potential deposited on the surface being charged.

Other corona discharge electrodes known to the art are intended to be included herein.

There also exist other means for charging a surface as, for example, a radioactive source of ions or the like, and such other means are intended to be included here- Although charging is illustrated as accomplished through the use of a corona discharge electrode which is moved relative to a plate, it is to be realized that stationary corona charging of a stationary plate is also intended to be encompassed by this invention. Stationary charging of a stationary plate may be accomplished through the use of a corona discharge electrode posi tioned above the top surface alone when the lower surface is positioned against a grounded electrode or above the top surface and beneath the rear surface of the photoconductive insulating layer when simultaneous charging of both surfaces is being carried out.

Illustrative of corona charging is a process; which utilizes a high A.C. potential supplied to the corona dis charge wires. In such a system, which will be described in connection with the technique illustrated in Fig. l, the shield is biased to the charge potential desired on the surface being charged. Thus, if it is desired to charge the top surface of the photoconductive insulating layer to a positive 600 volts, an A.C. corona generating potential is applied to the corona discharge wire and a positive limiting potential in the order of 600 volts D.C. is applied to the shield of the upper corona discharge electrode. An A.C. corona generating voltage is supplied to the discharge wires of the lower electrode and its shield is grounded. When the photoconductive insulating layerattains a charge of 60 0 volts the flow of corona current to the photoconductive insulating layer will stop and the rear surface of the photoconductive insulating layer would have been simultaneously charged to create the desired potential difference between the surfaces.

When charging is carried out as illustrated in Fig. 1, it is to be realized that a potential difference between the opposite surfaces is desired rather than necessarily potentials of opposite sign. Thus, it is feasible to charge one surface to plus 800 volts and the other surface to plus 300 volts. This may be accomplished using the A.C. corona charge means described above by biasing one shield at 800 volts D.C. and the other shield at 300 volts D.C. When charging is carried out as illustrated in Fig. 2, the potential diflerence is measured between the upper surface of the photoconductive insulating layer and the grounded support table.

The potential gradient desired through the photoconductive insulating layer will vary depending on a number of factors such as the thickness of the photoconductive insulating layer, insulating characteristics of the photoconductive insulating layer, dielectric strength of the photoconductive insulating layer, and the like. As a general proposition, however, it may be stated that valuable and usable reproductions maybe made in xerography when a potential difference of from volts exists through the photoconductive insulating layer. A potential difference of about 500 volts is generally preferred.

When charging is carried out as is described in connection with Fig. 2, the surface of the photoconductive insulating layer positioned against the support base is raised to a potential of the same sign as the potential being placed on the upper surface-of the photoconductive insulating layer. If during the process the plate is separated from the table top or the like, the potential on the rear surface of the plate tends to increase as separation increases. This results due .to a decrease in capacitance between the rear surface and ground because of the greater gap created during separation. A point of separation would likely be reached at which the potential would become sufficiently great to bring about a form of electric breakdown and transfer of charge to the rear surface of the plate member. However, if breakdown takes place, the charges that transfer would be substantially uniform in areas of charge transfer and would not distort a uniform charge on the upper surface of the photoconductive insulating layer or a charge pattern on the upper surface of the photoconductive insulating layer or in any way detrimentally affect image formation. The charges transferred in such an instance would result substantially in the charge pattern present when the techniques of charging illustrated in Fig. '1 are used. Thus, during the charging step in Fig. l, uniform and opposite charges are sprayed to the opposite surfaces ofthe self-supporting film of photoconductive insulating material. If separation of the plate from the support table illustrated in Fig. 2 takes place after the corona grid has passed across the surface, then a uniform amount of charge which is opposite in sign to the charge on the upper surface will transfer to the rear surface. Exposure of a charged plate charged according to the technique illustrated in Fig. 1 results in charge dissipation in areas struck by activating energy. Thus, as is illustrated in Fig. 4, in areas struck by light charge is dissipated, whereas in areas not struck by light the charges deposited during the charging step remain in position. Exposure of a plate charged according to the embodiment illustrated in Fig. 2 while the plate remains in position on the conductive support base results in charge migration to the under or rear surface of the photoconductive insulating layer. Charges remain at this lower surface due to the separation between the support table and the surface of the photoconductive insulating film. Generally, this separation will be a minute air gap which will provide a sufficient dielectric to maintain the charges at the lower surface of the photoconductive insulating film. However, if after exposure the plate is separated from the grounded support base, charges will move to the surface of the photoconductive insulating film due to air breakdown or the like as discussed above and, thus, charges at the lower surface will be neutralized, whereas charges which remained at the upper surface will be substantially balanced as in the case of exposure of the self-supporting film illustrated in Fig. 4.

Corona discharge electrodes are positioned at varying distances from the plate depending upon potentials supplied to the corona discharge wire or wires, potentials supplied to the shield, spacing of the discharge wire or wires from the shield, desired potential on the surface being charged, thickness of the corona discharge wire, or the like. Spacings of about inch between the discharge wires and the shield and about A to /2 inch between the discharge wires and the surface to be charged are generally used in xerography and have been found to work well with this invention.

The plate of this invention may comprise any of a number of materials as, for example, a self-supporting film of vitreous or amorphous selenium, anthracene, selenium-tellurium mixtures, sulphur, or photoconductive insulating materials in insulating binders, or the like. A presently preferred plate is a photoconductive insulating material in a resin binder and is fully described in copending application Serial No. 311,546 and comprises zinc oxide in a resin insulating binder. Various other photoconductive insulating layers comprising photoconductive insulating materials in insulating binders which may be used as the self-supporting film in this invention include, but are in no way limited to, zinc magnesium oxide, zinc sulfide, zinc cadmium sulfide, cadmium sulfide, cadmium strontium sulfide, zinc silicate, calcium tungstate, selenides and mixed selenides of cadmium and zinc, anthracene, titanium dioxide, and the like. Desirably, these materials are activated with small amounts, that is 0.01% to 0.001%, of metallic impurities, and is well known to those skilled in the art.

These materials are in some instances prepared immediately as a self-supporting film, whereas in other instances they are coated on a surface and then stripped 6 from the surface. One which has been made using the stripping technique comprised a layer of zinc oxide in a resin binder formed on a polyethylene coated surface and then stripped therefrom.

The use of the term ground throughout this application is intended to have the usual conventional meaning of a relative reference point. There is no desire for ground to limit this invention in any way to a specific value such as zero potential although in some instances fgroundmay indicate zero potential. Instead, ground" is intended to indicate a reference point from which other potential values vary upward when positive or downward when negative. 7

This invention by devising methods of image formation on plates comprising self-supporting films of material not backed by other materials substantially improves and broadens the art of xerography. It allows for the creation of fields of force which extend only through the photoconductive insulating layer. Such fields operate to create a sensitive plate for exposure and a plate which is able to produce images of high resolution. Further, when the charge pattern exists on both outer surfaces, as is possible with this invention, both surfaces may be developed thus allowing choice as to the direction of reading of the developed image or allowing for the formation of more than one copy at a time.

While the present invention as to objects and advantages, as has been described herein, has been carried out in specific embodiments thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly Within the spirit and scope of the appended claims.

What is claimed is:

l. The method, in electrostatic photography, of sensitizing a xerographic plate consisting of a self-supporting film of photoconductive insulating material, said method comprising, in darkness, positioning said plate between a pair of corona discharging sources located in spaced relationship facing and directly opposite to each other, applying to each of said corona discharging sources an A.C. electric potential to create corona discharge thereat, creating an electric field between said sources to cause ions of opposite polarity to move from. each of said sources to each facing surface of said plate by applying a potential difference to one of said discharge sources in respect to the other of said discharge sources, said applied electric potentials, applied to each of said sources to create corona discharge and applied bet-ween said sources to create an electric field therebetween being sufficiently intense to cause plate charging to above about volts when measuring one surface in respect to the other surface of said plate, and while said plate continues to be located between said corona discharging sources bringing about relative movement between said plate and said sources.

2. The method, in electrostatic photography, of forming an invisible electrostatic latent image on a xerographic plate consisting of a self-supporting film of photoconductive insulating material, said method comprising, in darkness, positioning said plate between a pair of corona discharge sources located in spaced relationship and directly opposite to each other, applying to each of said corona discharging sources an A.C. electric potential to create corona discharge thereat, creating an electric field between said sources to cause ions of opposite polarity to move from each of said sources to each facing surface of said plate by applying a potential difference to one of said discharge sources in respect to the other of said discharge sources, said applied electric potentials, applied to each of said sources to create corona discharge and applied between said sources to create an electric field therebetween being sufiiciently intense to cause plate charging to above about 150 volts when measuring one surface in respect to the other surface of said plate, bringing about relative movement between said plate and said sources, and following movement of said plate between said sources exposing said plate to a light image pattern to thereby form an electrostatic latent image of invisible charge on said plate conforming to said light image pattern.

3. The method of claim 1 in which said applied potential dilierence to create an electric field between said sources comprises a DC. potential difference.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, 1942 2,588,675 Walkup et al, Mar. 11, 1952 2,588,699 Carlson Mar. 11, 1952 8 Carlson Ian. 6, 1953 Middleton Dec. 22, 1953 Butterfield Nov. 2, 1954 Carlson Feb. 8, 1955 Young et a1. Jan. 31, 1956 McNaney June 15, 1957 Gundlach May 5, 1959 OTHER REFERENCES 10 Metcalfe et al.: Journal of the Oil & Colour Chem.

Assn., vol. 39, #11, pp. 845-856 (1958).

Young et al.:

R.C.A. Review, December 1954, pp.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,965,481 December '20 1960 Robert W, Gundlaoh It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent, should read as "corrected below. 7

Column 2, lines 62 and 63 for "mat i c" read plastic column 4L line 58,

Signed and sealed this 26th day of September 1961.

(SEAL) Attest:

ERNEST w. SWIDER DAVID L. Attesting Officer Commissioner of Patents USCOMM-DC- after "volts" insert E0 900 volts 

1. THE METHOD, IN ELECTROSTATIC PHOTOGRAPHY, OF SENSITIZING A XEROGRAPHIC PLATE CONSISTING OF A SELF-SUPPORTING FILM OF PHOTOCONDUCTIVE INSULATING MATERIAL, SAID METHOD COMPRISING, IN DARKNESS, POSITIONING SAID PLATE BETWEEN A PAIR OF CORONA DISCHARGING SOURCES LOCATED IN SPACED RELATIONSHIP FACING AND DIRECTLY OPPOSITE TO EACH OTHER, APPLYING TO EACH OF SAID CORONA DISCHARGING SOURCES AN A.C. ELECTRIC POTENTIAL TO CREATE CORONA DISCHARGE THEREAT, CREATING AN ELECTRIC FIELD BETWEEN SAID SOURCES TO CAUSE IONS OF OPPOSITE POLARITY TO MOVE FROM FROM EACH OF SAID SOURCES TO EACH FACING SURFACE OF SAID PLATE BY APPLYING A POTENTIAL DIFFERENCE TO ONE OF SAID DISCHARGE SOURCES IN RESPECT TO THE OTHER OF SAID DISCHARGE SOURCES, SAID APPLIED ELECTRIC POTENTIALS, APPLIED TO EACH OF SAID SOURCES TO CREATE CORONA DISCHARGE AND APPLIED BETWEEN SAID SOURCES TO CREATE AN ELECTRIC FIELD THEREBETWEEN BEING SUFFICIENTLY INTENSE TO CAUSE PLATE CHARGING TO ABOVE ABOUT 150 VOLTS WHEN MEASURING ONE SURFACE IN RESPECT TO THE OTHER SURFACE OF SAID PLATE, AND WHILE SAID PLATE CONTINUES TO BE LOCATED BETWEEN SAID CARONA DISCHARGING SOURCES BRINGING ABOUT RELATIVE MOVEMENT BETWEEN SAID PLATE AND SAID SOURCES. 